Service providers (SPs) are seeking an efficient, scalable, yet simple means of distributing multicast data. There are a number of different approaches with trade-offs for each. Historically, the SP has either provided basic transport for multicast data where the subscriber is responsible for the replication or the SP provides multicast services where replication is done on behalf of the subscriber. The former model is very inefficient, as the subscriber must replicate the data onto each transport instance built in the SP; nevertheless, the architecture is simple for the provider to support. In contrast, the multicast service is much more efficient; however, it is much more complex for the SP to operate. In both cases, scalability is constrained by bandwidth efficiency or operational complexity, respectively. An ideal solution leverages the concepts of both the multicast transport and the multicast service mitigating the extreme compromises of either solution.
Psuedo wires (PW) are point-to-point (P2P) tunnels that carry Layer 2 (L2) Packet Data Units (PDU)s across a packet switched network (PSN). The PW is set up using directed Label Distribution Protocol (LDP) sessions between two provider edge (PE) routers and the one or more PWs are carried inside point-to-point (P2P) PSN tunnel Label Switched Paths (LSP)s (independently established using PSN tunnel signaling protocols) between the same two PE nodes. The directed LDP session is used to negotiate/advertise the PW parameters and the PW label. When an ingress PE router receives an L2 PDU, it pushes a PW label and then a PSN tunnel label on the MPLS stack. The PSN tunnel label is used to transport the MPLS packet to the egress PE router. The egress PE router uses the PW label to identify which outbound L2 interface the L2 PDU should be forwarded to.
Some applications require L2 PDUs to be replicated from one or more ingress (source) PE routers to multiple egress (target) PE routers. An example of such an application is video distribution. Given the point-to-point nature of PWs and PSN tunnels and the independent signaling machinery used for each, the only practical technique in use today to accomplish the one-to-many or many-to-many L2 multipoint behavior is to first establish P2P PSN tunnels from each source PE router out to N number of target PE routers (using PSN tunnel signaling protocols) and second, use directed LDP sessions to build the P2P PWs from the same source PE router out to the same N number of target PE routers.